1. Field of the Invention
This invention relates to a print head for use in dot matrix printers, and more particularly to a print head for use in dot matrix printers for moving and impacting print styli onto recording paper against a platen by an armature electromagnetically driven by an electromagnet.
2. Description of the Related Arts
Generally, a dot matrix printer which is capable of forming characters, numerals, and other symbols on recording paper by the use of a plurality of dots has been widely known and has been used recently in various fields of industry in printers of data processing system.
U.S. Pat. No. 4,986,179 owned by the inventors of this application discloses a print head for use in such printers, and FIG. 3 of the accompanying drawings is a cross sectional plan view showing the configuration of a stylus actuator assembly comprised in the print head. In FIG. 3, a stylus actuator assembly 20 is placed in position on a head frame 22, that is, a position where the stylus actuator assembly 20 can give impact to recording paper against a platen "P", and screwed with a fixing screw 30 provided in an electromagnetic actuator 28 which is composed of a core 24 with a coil 26 wound therearound.
An armature 32 is attached to the central portion of a torsion leaf spring 34 having torsional property, and the torsion leaf spring 34 is fixed to the stylus actuator assembly 20, thereby supporting the armature 32.
On the top side of a support rack portion 38 fixedly positioned by both an electromagnetic actuator 28 and a yoke base 36 mounted to the actuator 28, a torsion leaf spring rest 40 is provided. The torsion leaf spring 34 is placed in position on this torsion leaf spring rest 40 and secured thereon with a fixing bolt 44 through a spring presser 42.
The armature 32 is mounted to a torsion spring 34 by means of a rivet, brazing, or spot welding.
At one end, the armature 32 has a stylus holder 48 for holding a stylus 46, confronting a platen roller. At the other end, opposite to the position of the stylus 46, the armature 32 has a plunger 50 attracted by electromagnetic force exerted from a coil 26 of the electromagnetic actuator 28.
At the support rack portion 38 is fixed with a screw 54, confronting the stylus holder 48, a damper 52 which absorbs impact caused when the stylus 46 is forcibly restored to the original position by the torsional force of a torsion spring 34.
As shown in FIG. 5A and 5B, the armature 32 has a bent portion 32b, which is bent substantially orthogonally to a base portion 32a, on its right side relative to an imaginary line drawn from the plunger 50 to the stylus 46. This bent portion 32b provides the armature 32 with rigidity.
FIG. 4 shows an arrangement in which the stylus actuator assemblies 20 are mounted on the head frame 22. A plurality of stylus actuator assemblies 20 are arranged one after the other in two rows in such a manner that a stylus actuator assembly in one row confronts a stylus actuator assembly in the other row adjacently and with all of the styli 46 in a line.
Since the armatures of the stylus actuator assemblies 20 in both rows have the same shape, it can be said that the bent portions 32b of the armatures 32 in the lower row are situated on a backward side thereof whereas the those on the armatures in the upper row are situated on an advancing side thereof.
Referring to FIG. 3, the operation of the print head will be described hereinbelow.
When printing on recording paper, a predetermined impact driving current is supplied to the electromagnetic actuator 28, the core 24 is excited, and then the plunger 50 of the armature 32 is attracted by electromagnetic force in the direction of the arrow 100.
At this time, the armature 32 moves pivotally around the torsion spring 34 center, the stylus 46 supported at one end of the armature 32 is driven and impacted in the direction of the arrow 200 opposite to the direction in which the plunger 50 on the other end of the armature 32 is attracted. Thus, dot printing is effected by the tip end portion of the stylus 46.
However, according to the conventional print head for dot matrix printers, since the armatures having the same structure are arranged in both upper and lower rows, the direction of impact displacements of the armatures is different between the upper row and the lower row because the torsional force of the armature 32 produced by rigidity thereof may differ depending on the location of the bent portion relative to the traversing direction of the print head. The rigidity of the armature may be differed depending on the position of the bent portion 32b relative to the head traveling direction.
The result of this is that dots are formed unevenly spaced from each other in any traverse direction thereto, and this uneven spacing of dots due to variations of the torsional force among styli is discernible and detrimental to print quality. Therefore, there has been a demand for preventing such unevenness of dots and obtaining an arrangement which forms evenly spaced dots.